In industrial measurements technology, especially also in connection with the control and monitoring of automated manufacturing processes, for ascertaining characteristic measured variables of media flowing in a process line, for example, a pipeline, for example, of media such as liquids and/or gases, often such measuring systems are used, which, by means of a measuring transducer of vibration-type and a transmitter electronics connected thereto, most often a transmitter electronics accommodated in a separate electronics housing, induce in the flowing medium reaction forces, for example, Coriolis forces, and produce, repetitively derived from these, measured values, for example, mass flow rate, density, viscosity or some other process parameter correspondingly representing the at least one measured variable. Such measuring systems—often formed by means of an in-line measuring device in compact construction with integrated measuring transducer, such as, for instance, a Coriolis, mass flow meter, —are long known and have proven themselves in industrial use. Examples of such measuring systems with a measuring transducer of vibration-type or also individual components thereof, are described e.g. in EP-A 421 812, EP-A 462 711, EP-A 763 720, EP-A 1 248 084, U.S. Pat. No. 4,680,974, U.S. Pat. No. 4,738,144, U.S. Pat. No. 4,768,384, U.S. Pat. No. 4,801,897, U.S. Pat. No. 4,823,614, U.S. Pat. No. 4,879,911, U.S. Pat. No. 5,009,109, U.S. Pat. No. 5,050,439, U.S. Pat. No. 5,359,881, U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,610,342, U.S. Pat. No. 5,734,112, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,926,096, U.S. Pat. No. 5,969,264, U.S. Pat. No. 7,127,952, U.S. Pat. No. 6,092,429, U.S. Pat. No. 6,311,136, U.S. Pat. No. 6,883,387, U.S. Pat. No. 7,325,461, U.S. Pat. No. 7,392,709, U.S. Pat. No. 7,421,350, U.S. Pat. No. 7,610,795, U.S. Pat. No. 2010/0050783, US-A 2010/0251830, US-A 2010/0242623, WO-A 96/08697, WO-A 98/40702, WO-A 2004/099735, WO-A 2005/050145, WO-A 2007/040468, WO-A 2008/059015, WO-A 2010/059157 or the not pre-published German patent application DE 102009046043.8 of Endress+Hauser.
Therein shown measuring transducers comprise at least two, essentially straight, or curved, e.g. U-, or V-shaped, equally constructed, measuring tubes accommodated in a measuring transducer housing for conveying the medium, in given cases, a medium that is also inhomogeneous, extremely hot and even very viscous. The at least two measuring tubes can, as, for example, shown in the mentioned U.S. Pat. No. 5,734,112, U.S. Pat. No. 5,796,011 or US-A 2010/0242623, be integrated into the process line via a flow divider extending on the inlet side between the measuring tubes and an inlet-side connecting flange as well as via a flow divider extending on the outlet side between the measuring tubes and an outlet-side connecting flange, in order to form a tube arrangement with flow paths connected in parallel with one another. The measuring tubes can, however, also, as shown, for example, in the mentioned EP-A 421 812, EP-A 462 711, EP-A 763 720, be integrated into the process line via in- and outlet tube pieces, in order to form a tube arrangement with a single traversing flow path. In measurement operation, the measuring tubes, which are flowed through—in parallel, or serially—are then caused to vibrate for the purpose of generating oscillation forms influenced by the medium flowing through.
Selected as excited oscillation form—the so-called wanted mode—is, in the case of measuring transducers with curved measuring tubes, usually that eigenoscillation form (eigenmode), in the case of which each of the measuring tubes moves as a cantilever in a pendulum-like manner at least partially at a natural resonance frequency (eigenfrequency) about an imaginary longitudinal axis of the measuring transducer, whereby Coriolis forces are induced in the through flowing medium as a function of the mass flow. These, in turn, lead to the fact that superimposed on the excited oscillations of the wanted mode, in the case of curved measuring tubes, thus the pendulum-like, cantilever oscillations, are thereto equal frequency, bending oscillations according to at least one likewise natural, second oscillation form of, in comparison to the wanted mode, higher (modal) order, the so-called Coriolis mode. In the case of measuring transducers with curved measuring tubes, these cantilever oscillations in the Coriolis mode forced by Coriolis forces correspond usually to that eigenoscillation form, in the case of which the measuring tube also executes rotary oscillations about an imaginary vertical axis directed perpendicularly to the longitudinal axis. In the case of measuring transducers with straight measuring tubes, in contrast, for the purpose of producing mass flow dependent, Coriolis forces, often a wanted mode is selected, in the case of which each of the measuring tubes execute, at least partially, bending oscillations essentially in a single imaginary plane of oscillation, so that the oscillations in the Coriolis mode, accordingly, are embodied as bending oscillations of equal oscillation frequency coplanar with the wanted mode oscillations.
For active exciting of oscillations of the at least two measuring tubes, measuring transducers of the vibration-type have, additionally, an exciter mechanism driven, during operation, by an electrical driver signal, e.g. a controlled electrical current, generated and correspondingly conditioned by the mentioned transmitter electronics, and, respectively, a therein correspondingly provided, special driver circuit. The exciter mechanism excites the measuring tube by means of at least one electromechanical, especially electrodynamic, oscillation exciter flowed through during operation by an electrical current and acting practically directly, especially differentially, on the at least two measuring tubes, such that they execute bending oscillations, especially opposite equal, bending oscillations, in the wanted mode. Furthermore, such measuring transducers include a sensor arrangement with oscillation sensors, especially electrodynamic, oscillation sensors, for the at least pointwise registering of inlet-side and outlet-side oscillations of at least one of the measuring tubes, especially opposite equal bending oscillations of the measuring tubes in the Coriolis mode, and for producing electrical sensor signals serving as vibration signals of the measuring transducer and influenced by the process parameter to be registered, such as, for instance, the mass flow or the density. As described, for example, in U.S. Pat. No. 7,325,461, in the case of measuring transducers of the type being discussed, in given cases, also the oscillation exciter can, at least at times, be used as oscillation sensor and/or an oscillation sensor at least at times as oscillation exciter. The exciter mechanism of measuring transducers of the type being discussed includes usually at least one electrodynamic oscillation exciter and/or an oscillation exciter acting differentially on the measuring tubes, while the sensor arrangement comprises an inlet-side, most often likewise electrodynamic, oscillation sensor as well as at least one thereto essentially equally constructed, outlet-side oscillation sensor. Such electrodynamic and/or differential oscillation exciters of usually marketed measuring transducers of vibration-type are formed by means of a magnet coil, through which electrical current flows, at least at times, and which is affixed on one of the measuring tubes, as well as by means of a permanent magnet interacting with the at least one magnet coil, especially plunging into such, serving as a rather elongated armature, especially with rod-shaped form, correspondingly affixed on the other, opposite equally moving, measuring tube. The permanent magnet and the magnet coil serving as exciter coil are, in such case, usually so oriented that they extend essentially coaxially relative to one another. Additionally, in the case of conventional measuring transducers, the exciter mechanism is usually embodied in such a manner and placed in the measuring transducer such that it acts essentially centrally on the measuring tubes. In such case, the oscillation exciter and, insofar, the exciter mechanism, is, as shown, for example, also in the case of that in the proposed measuring transducers, affixed outwardly on the respective measuring tube at least pointwise along an imaginary central peripheral line thereof. Alternatively to an exciter mechanism formed by means of oscillation exciters acting rather centrally and directly on the respective measuring tube, the exciter mechanism can, as, among other things, provided in U.S. Pat. No. 6,092,429 or U.S. Pat. No. 4,823,614, for example, also be formed by means of two oscillation exciters affixed, in each case, not in the center of the respective measuring tube, but, instead rather on the in-, and, respectively, outlet sides.
In the case of usually marketed measuring transducers of vibration-type, the oscillation sensors of the sensor arrangement are, at least insofar as they work according to the same principle of action, embodied with essentially the same construction as the at least one oscillation exciter. Accordingly, also the oscillation sensors of such a sensor arrangement are most often, in each case, formed by means of at least one magnet coil, which is affixed on one of the measuring tubes and, at least at times, passed through by a variable magnetic field and, associated therewith, supplied, at least at times, with an induced measurement voltage, as well as a permanently magnetic armature, which delivers the magnetic field, with the armature being affixed on another of the measuring tubes and interacting with the at least one coil. Each of the aforementioned coils is additionally connected by means of at least one pair of electrical connecting lines with the mentioned transmitter electronics of the in-line measuring device. These electrical connecting lines are led most often on as short as possible paths from the coils to the measuring transducer housing. Due to the superimposing of wanted—and Coriolis modes, the oscillations of the vibrating measuring tubes registered by means of the sensor arrangement on the inlet side and on the outlet side have a measurable phase difference also dependent on the mass flow. Usually, the measuring tubes of such measuring transducers applied, e.g. in Coriolis, mass flow meters are excited during operation to an instantaneous natural resonance frequency of the oscillation form selected for the wanted mode, e.g. at constant controlled oscillation amplitude. Since this resonance frequency depends, especially, also on the instantaneous density of the medium, market-usual Coriolis, mass flow meters can measure, besides mass flow, supplementally also the density of flowing media. Additionally, it is also possible, as, for example, shown in U.S. Pat. No. 6,651,513 or U.S. Pat. No. 7,080,564, by means of measuring transducers of vibration-type, directly to measure viscosity of the through flowing medium, for example, based on the exciter energy, or excitation power, required for maintaining the oscillations and/or based on the attenuation, or damping, of oscillations of the at least one measuring tube, especially oscillations in the aforementioned wanted mode, resulting from dissipation of oscillatory energy. Moreover, also other measured variables, such as, for instance, according to U.S. Pat. No. 6,513,393, the Reynolds number, derived from the aforementioned primary measured values, mass flow rate, density and viscosity, can be ascertained.
In the case of measuring transducers of the type being discussed, it is especially important to trim the oscillation characteristics of individual measuring transducer components, not last also the at least one measuring tube, consequently the said oscillation characteristics characterizing, or influencing, parameters, such as, for instance, tube shapes, and, respectively, cross sections, tube wall thicknesses and, associated therewith, mass distributions, bending stiffnesses, eigenfrequencies etc., of each individual measuring transducer example as exactly as possible at a target dimension therefor, in each case, predetermined for defined reference conditions, and, respectively, to hold the scattering of said parameters within a population of manufactured measuring transducer of such type in an as narrow as possible tolerance range predetermined therefor. Equally important in the case of measuring transducers of the type being discussed is to prevent possible imbalances of the respective tube arrangement, brought about, for instance, by non-uniform, consequently non-symmetric, mass and/or stiffness distributions within the tube arrangement.
In such case, it is, among other things, also of special interest, at an, as much as possible, “late” production phase, to set the eigenfrequencies of the respective tube arrangement of the measuring transducer to the desired target(s), here thus one or more selected target eigenfrequencies, respectively, correspondingly to compensate possible imbalances, in order to be able to prevent, reliably, possible newer detunings of the tube arrangement in a following production phase of the measuring transducer.
In the initially mentioned U.S. Pat. No. 5,610,342, for example, a method for the dynamic tuning of a tube serving as measuring tube of a measuring transducer of vibration-type to a target stiffness is shown, in the case of which method the tube is pressed in on its two tube ends in bores of a first, and, respectively, second end piece of a support tube by targeted plastic deformation of the tube walls in the region of the tube ends and the entire tube arrangement is simultaneously adjusted to a target eigenfrequency. Additionally, in the initially mentioned U.S. Pat. No. 7,610,795, a method is described for tuning a tube serving as measuring tube of a measuring transducer of vibration-type to a target eigenfrequency, consequently to a target bending stiffness co-determined by the tube geometry and cross section, by means of a fluid introduced therein and supplied with an (over-) pressure introducing plastic deformation of at least of a part of its tube wall.
A disadvantage of the methods known from the state of the art is, among other things, that they are very complicated. Moreover, another disadvantage of the aforementioned methods is that, inherently therewith, ultimately a certain change of the geometry of the tubes, namely a deviation from the ideal circular shape of the cross section, or an increased deviation from perfect homogeneity of the cross section in the longitudinal direction, consequently a deviation of the contour of the lumen of the tube from the ideal form, is introduced.